Absorbing, reflecting and collimating polarizer stack and backlights incorporating same

ABSTRACT

Polarizer stacks are described. More particularly, polarizer stacks that include an absorbing polarizer and multiple reflective polarizers, including at least one collimating reflective polarizer are described. Such polarizer stacks are capable of emitted light that is both collimated and color neutral. Backlights incorporating such polarizer stacks are also described.

FIELD

The present description relates to polarizer stacks. More particularly,the present description relates to polarizer stacks that include bothreflective and absorbing polarizers, and provide light output that iscollimated and color neutral. Additionally backlights that incorporatesuch polarizer stacks are described.

BACKGROUND

Display devices, such as liquid crystal display (LCD) devices, are usedin a variety of applications including, for example, televisions,hand-held devices, digital still cameras, video cameras, and computermonitors. Because an LCD panel is not self-illuminating, some displayapplications may require a backlighting assembly or a “backlight.” Abacklight typically couples light from one or more sources (e.g., a coldcathode fluorescent tube (CCFT) or light emitting diodes (LEDs)) to theLCD panel. Common display devices usually include polarizers.

Reflective polarizers are known. Often times, reflective polarizers maybe made up of multilayer optical films that incorporate a large numberof thin layers of different light transmissive materials, often referredto as microlayers because they are thin enough so that the reflectionand transmission characteristics of the optical film are determined inlarge part by constructive and destructive interference of lightreflected from the layer interfaces. In such reflective polarizers, aplurality of microlayers' in-plane refractive indices are selected toprovide a substantial refractive index mismatch between adjacentmicrolayers along an in-plane block axis and a substantial refractiveindex match between adjacent microlayers along an inplane pass axis.Such films are described in, e.g., U.S. Pat. No. 5,882,774 (Jonza etal.). Such reflective polarizers have a sufficient number of layers toensure high reflectivity for normally incident light polarized along theblock axis, while maintaining low reflectivity and high transmission fornormally incident light polarized along the pass axis.

In some display applications, a reflective polarizer may be combinedwith, e.g. an absorbing polarizer. Such constructions are described in,e.g., U.S. Pat. No. 6,096,375 to Ouderkirk et. al., U.S. Pat. No.6,697,195 to Weber et. al. and U.S. Pat. No. 7,826,009 to Weber et al.,each of which is hereby incorporated by reference in its entirety.Absorbing polarizers are made, for example, by incorporating a dye intoa polymer sheet that is then stretched in one direction. Absorbingpolarizers can also be made by uniaxially stretching a semicrystallinepolymer such as polyvinyl alcohol, then staining the polymer with aniodine complex or dichroic dye, or by coating a polymer with an orienteddichroic dye. Many commercial polarizers typically use polyvinyl alcoholas the polymer matrix for the dye. Absorbing polarizers normally have alarge amount of absorption of light (and may also in some literature bereferred to as “dichroic polarizers”). The use of such “hybridpolarizers” (combining reflective and absorbing polarizers) can providevery good extinction if the extinction spectra of the layers arecarefully designed.

Further, 3M Company has recently developed reflective polarizers thatcan combine light collimating functions generally provided by structuredfilms, such as brightness enhancing prismatic or beaded films, whilestill providing the requisite reflective polarization function. Suchreflective polarizers are described, e.g., in commonly owned andassigned International Application Nos. PCT/US2012/060485 andPCT/US2012/060483, each of which is hereby incorporated by reference inits entirety. The films described in these applications provide uniqueshapes of transmission spectra in order to balance reflected andtransmitted color, and provide a neutral white display, while alsocollimating light generally towards a viewer. As a result of thesespectral designs for neutral white backlight output color, theextinction state of light of the reflective polarizer often has anon-neutral color transmitting through the film that must then beneutralized by a separate high performance iodine absorbing polarizerelement (prior to reaching the liquid crystal panel). Such a separateelement has also been necessary as the block axis transmission levels ofthe reflective polarizer films have been too high to act as the onlypolarizing element immediately adjacent a liquid crystal panel, and evenwhere the overall transmission level in the extinction state aregenerally low, there may be spikes in one area of the color spectrum.

There remains a need in the art for a singular film stack that acts as apolarizer and serves the properties of reflection, absorption andcollimation, while also providing for color neutrality in both the passand extinction states. The present description seeks to address thisneed.

SUMMARY

In one aspect, the present description relates to a polarizer stack. Thepolarizer stack includes a first birefringent reflective polarizerhaving pass and block axis transmission spectra, a collimatingbirefringent reflective polarizer having a block axis transmittance thatdecreases with increasing wavelength, and an absorbing polarizer layerpositioned between the first birefringent reflective polarizer andcollimating birefringent reflective polarizer. The pass axistransmission of the polarizer stack as a whole is substantially neutralacross the visible wavelength band. The polarizer stack may also includea second absorbing polarizer layer positioned on the opposite side ofthe first birefringent polarizer from the absorbing polarizer layer.

In some embodiments, the block axis transmittance of the firstbirefringent polarizer increases as wavelength increases across thevisible spectrum. In some embodiments, the pass axis transmittance ofthe collimating reflective polarizer is neutral or decreases aswavelength increases across the visible spectrum. In some embodiments,the polarizer stack satisfies: T^(pass)60/T^(pass)0<0.75 orT^(pass)60/T^(pass)0<0.60 for p-pol or potentially for s-pol light.Additionally, the T^(pass) of visible light may be greater than 0.3, 0.4or 0.5. Further the polarizer stack may satisfy T^(block)0<10⁻³.

In some embodiments, the contrast ratio of the absorbing polarizer layeris 100:1 or less. In some, the contrast ratio of the polarizer stack is6,000:1 or more. Additionally, the R_(hemi), of first birefringentpolarizer may be <0.50, and the R_(hemi) of the collimating birefringentreflective polarizer may be at least 0.60.

In a different aspect, the present description relates to a backlightthat includes a light source and the polarizer stack previouslydescribed. Additionally, the present description may relate to abacklight that includes a display and the backlight described.

In another aspect, the present description relates to a backlight. Thebacklight includes (1) a light recycling cavity, the light cavitycomprising: a front reflector, a back reflector, a Pass IntensitySpectrum and a Block Intensity Spectrum, wherein the front reflector ispartially reflective and includes an ARCP; and (2) one or more lightsource members disposed to emit light into the light recycling cavity.The Pass Intensity Spectrum and Block Intensity Spectrum are bothsubstantially neutral across a visible wavelength band and have theratio of at least 500:1 at normal incidence. In at least someembodiments, the ARCP includes a first birefringent reflective polarizerhaving pass and block axis transmission spectra, a collimatingbirefringent reflective polarizer having a block axis transmittance thatdecreases with increasing wavelength, and an absorbing polarizer layerpositioned between the first birefringent reflective polarizer andcollimating birefringent reflective polarizer.

In some embodiments the pass axis transmission of the ARCP issubstantially neutral across the visible wavelength band. Additionally,the Pass Intensity Spectrum and Block Intensity Spectrum may both besubstantially neutral across a visible wavelength band and have theratio of at least 1,000:1 at normal incidence. Further, the ARCP maysatisfy: T^(pass)60/T^(pass)0<0.75, or T^(pass)60/T^(pass)0<0.60 forp-pol or s-pol. light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a polarizer stack according to thepresent description;

FIG. 2 is a is a schematic perspective view a reflective polarizingfilm;

FIG. 3 is a schematic perspective view of a reflective polarizeraccording to the present disclosure;

FIG. 4 is a cross-sectional view of a polarizer stack according to thepresent description;

FIG. 5 is a graph of luminance data versus polarizer angle for variouspolarizer stacks.

FIG. 6 is a graph of luminance data versus polarizer angle for variouspolarizer stacks.

FIG. 7 is a cross-sectional view of a display system with a backlightand LC panel.

FIGS. 8 a-d illustrate transmission spectra of polarizers according tothe present description.

FIG. 9 illustrates transmission spectra of an ARCP of the presentdescription versus a standard absorbing polarizer.

FIG. 10 illustrates the spectra of R_(BL)(λ) for a backlight reflectorand of ARCP R^(f) _(hemi)(λ) as well as pass intensity spectra.

FIGS. 11 a-b illustrate chromacity data for light emitted from abacklight with an ARCP front reflector.

FIG. 12 illustrates the computed transmission spectra for an ARCPaccording to the present description.

FIG. 13 illustrates the spectra of R_(BL)(λ) for a backlight reflectorand of ARCP R^(f) _(hemi)(λ) as well as pass intensity spectra.

FIGS. 14 a-b illustrate chromacity data for light emitted from abacklight with an ARCP front reflector.

FIG. 15 illustrates the computed transmission spectra for an ARCPaccording to the present description.

FIG. 16 illustrates the spectra of R_(BL)(λ) for a backlight reflectorand of ARCP R^(f) _(hemi)(λ) as well as pass intensity spectra.

FIGS. 17 a-b illustrate chromacity data for light emitted from abacklight with an ARCP front reflector.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which are shown byway of illustration several specific embodiments of devices, systems andmethods. It is to be understood that other embodiments are contemplatedand may be made without departing from the scope or spirit of thepresent disclosure. The following detailed description, therefore, isnot to be taken in a limiting sense.

All scientific and technical terms used herein have meanings commonlyused in the art unless otherwise specified. The definitions providedherein are to facilitate understanding of certain terms used frequentlyherein and are not meant to limit the scope of the present disclosure.

The present description provides for a polarizer stack that acts as anabsorbing, reflective, and collimating polarizer. The polarizer stackcan provide higher brightness than a conventional dichroic polarizerused with a liquid crystal backlight display and can also collimatelight towards a normal-angle viewer while maintaining neutral color inboth bright and dark states. The use of dichroic layers in the stackallow for neutral color with multilayer optical films that otherwisewould have objectionable color. Such a polarizer stack is highly usefulin displays including liquid crystal displays, touch screen displays,transflective displays, for phone, mobile, tablet, notebook, monitor andthe like applications. The unique combination of functions noted in asingular stack allows for thinner, simpler and more efficient displays.

In another sense, the present description provides a solution to theproblem of creating a polarizing element for use with a Liquid CrystalDisplay and associated backlight that provides a black dark or blockstate, and neutral white or pass state without the necessity of aconventional PVA Iodine polarizer used immediately adjacent to the LCpanel.

Referring to FIG. 1, the polarizer stack 100 includes a firstbirefringent reflective polarizer 106, an absorbing polarizer 104 and acollimating birefringent reflective polarizer 102 that is positioned onthe opposite side of the absorbing polarizer from first birefringentpolarizer 106. As noted above, the present description provides for thecollimation of light, and generally this is achieved through collimatingbirefringent reflective polarizer 102. The polarizer stack 100,simultaneously is absorbing, reflecting, collimating and polarizing, andis referred to below as either the polarizer stack or an absorbing,reflective, collimating polarizer (“ARCP”). The two terms should beunderstood to have the same meaning. The general function of collimatingbirefringent reflective polarizer may be better understood through thefollowing description of collimating birefringent reflective polarizer102.

In many embodiments, the collimating birefringent reflective polarizerhas a reflectivity that generally increases with angle of incidence, anda transmission that generally decreases with angle of incidence, wherethe reflectivity and transmission are for unpolarized visible light andfor any plane of incidence, and/or for light of a useable polarizationstate incident in a plane for which oblique light of the useablepolarization state is p-polarized, for one plane of reference, ands-polarized for the orthogonal plane of reference. Further, thecollimating birefringent reflective polarizer preferably has a highvalue of hemispheric reflectivity R^(f) _(hemi), while also having asufficiently high transmission of application-useable light.

In many embodiments, collimating birefringent reflective polarizers havea relatively high overall reflectivity to support relatively highrecycling within a cavity or film stack. We characterize this in termsof “hemispheric reflectivity,” meaning the total reflectivity of acomponent (whether a surface, film, or collection of films) when light(of a wavelength range of interest) is incident on it across adistribution of all possible directions. Thus, the component isilluminated with light incident from all directions (and allpolarization states, unless otherwise specified) within a hemispherecentered about a normal direction, and all light reflected into thatsame hemisphere is collected. The ratio of the total flux of thereflected light to the total flux of the incident light for thewavelength range of interest yields the hemispheric reflectivity,R_(hemi). Of particular note, R_(hemi) is characterized at discretewavelengths, and may be taken as an average-value across a range ofwavelengths of interest. Further, unlike the reflectivity for normalincidence, R_(hemi) is insensitive to, and already takes into account,the variability of reflectivity with incidence angle, which may be verysignificant for some components (e.g., prismatic films).

In fact, some embodiments of collimating birefringent reflectivepolarizers exhibit a (direction-specific) reflectivity that increaseswith incidence angle away from the normal (and a transmission thatgenerally decreases with angle of incidence), at least for lightincident in one plane. Such reflective properties cause the light to bepreferentially transmitted from the cavity or stack in which thecollimating polarizer resides, out through the collimating birefringentreflective polarizer at angles closer to the normal, i.e., closer to theviewing axis of the backlight, and this helps to increase the perceivedbrightness of the display at viewing angles that are important in thedisplay industry (at the expense of lower perceived brightness at higherviewing angles, which are usually less important). This effect is termedcollimation. We say that the increasing reflectivity with angle behavioris “at least for light incident in one plane,” because sometimes anarrow viewing angle (more collimation) is desired for only one viewingplane, and a wider viewing angle (less collimation) is desired in theorthogonal plane. An example is some LCD TV applications, where a wideviewing angle is desired for viewing in the horizontal plane, but anarrower viewing angle is specified for the vertical plane. In othercases, narrow angle viewing is desirable in both orthogonal planes so asto maximize on-axis brightness. In this manner, light from a recyclingcavity or film stack can be collimated to a significant degree, and apolarized light output from a single film construction can be provided.

In the following discussion of reflectivity and transmissioncharacteristics we can initially assume reflectivity and transmissionare determined as broad averages across a range of wavelengths. Laterdiscussion will focus on sloped transmission and reflection spectraacross the visible band, and reflectivity and transmissioncharacteristics are wavelength dependent and need to be characterizedwithin specific wavelength regions.

As we discuss oblique angle reflectivity, it is helpful to keep in mindthe geometrical considerations of FIG. 2. There, we see a surface 110that lies in an x-y plane, with a z-axis normal direction. As thesurface is a polarizing film or partially polarizing film (such as thosedescribed in International Application Nos. PCT/US2012/060485 andPCT/US2012/060483) we designate for purposes of this application they-axis as the “pass axis” and the x-axis as the “block axis.” In otherwords, normally incident light whose polarization axis is parallel tothe y-axis is preferentially transmitted compared to normally incidentlight whose polarization axis is parallel to the x-axis.

Light can be incident on surface 110 from any direction, but weconcentrate on a first plane of incidence 132, parallel to the x-zplane, and a second plane of incidence 122, parallel to the y-z plane.“Plane of incidence” refers to a plane containing the surface normal anda particular direction of light propagation. We show in FIG. 2 oneoblique light ray 130 incident in the plane 132, and another obliquelight ray 120 incident in the plane 122. Assuming the light rays to beunpolarized, they will each have a polarization component that lies intheir respective planes of incidence (referred to as “p-polarized”light), and an orthogonal polarization component that is orientedperpendicular to the respective plane of incidence (referred to as“s-polarized light” and labeled “s” in FIG. 2). It is important to notethat for reflecting polarizing films, “s” and “p” can be aligned witheither the pass axis or the block axis, depending on the direction ofthe light ray. In FIG. 2, the s-polarization component of ray 130, andthe p-polarization component of ray 120, are aligned with the pass axis(the y-axis) and thus would be preferentially transmitted, while theopposite polarization components (p-polarization of ray 130, ands-polarization of ray 120) are aligned with the block axis (x-axis).

With this in mind, let us consider the meaning and consequences ofspecifying that the collimating birefringent reflective polarizer“exhibits a reflectivity that generally increases with angle ofincidence.” This reflective property, used in conjunction with arecycling backlight is generally described in commonly owned andassigned U.S. Patent Publication No. 2010/0156953, and No. 2010/0165660.Collimating birefringent reflective polarizers with enhanced collimatingproperties are further described in PCT Application No.PCT/US2012/060485 wherein the reflection spectra are sloped. For aninstance where a light of a preferred polarization is desirable, thecollimating birefringent reflective polarizer includes a multilayerconstruction (e.g., coextruded polymer microlayers that have beenoriented under suitable conditions to produce desired refractive indexrelationships and desired reflectivity characteristics) having a veryhigh reflectivity for normally incident light in the block polarizationstate and a lower but still substantial reflectivity (e.g., 20% to 90%)for normally incident light in the pass polarization state. The veryhigh reflectivity of block-state light (p-polarized component of ray130, and s-polarized component of ray 120) generally remains very highfor all incidence angles. The more interesting behavior is for thepass-state light (s-polarized component of ray 130, and p-polarizedcomponent of ray 120), since that exhibits an intermediate reflectivityat normal incidence. Oblique pass-state light in the plane of incidence132 will exhibit an increasing reflectivity with increasing incidenceangle due to the nature of s-polarized light reflectivity (the relativeamount of increase, however, will depend on the initial value ofpass-state reflectivity at normal incidence). Thus, light emitted from arecycling backlight with a collimating birefringent reflective polarizerfront film in a viewing plane parallel to plane 132 will be partiallycollimated or confined in angle. Oblique pass-state light in the otherplane of incidence 122 (i.e., the p-polarized component of ray 120),however, can exhibit any of three behaviors depending on the magnitudeand polarity of the z-axis refractive index difference betweenmicrolayers relative to the in-plane refractive index differences. See,e.g., U.S. Pat. No. 5,882,774.

In one case, a Brewster angle exists, and the reflectivity of this lightdecreases with increasing incidence angle. This produces bright off-axislobes in a viewing plane parallel to plane 122, which are usuallyundesirable in LCD viewing applications (although in other applicationsthis behavior may be acceptable, and even in the case of LCD viewingapplications this lobed output may be redirected towards the viewingaxis with the use of a prismatic film and such).

In another case, a Brewster angle does not exist or is very large, andthe reflectivity of the p-polarized light is relatively constant withincreasing incidence angle. This produces a relatively wide viewingangle in the referenced viewing plane.

In the third case, no Brewster angle exists, and the reflectivity of thep-polarized light increases significantly with incidence angle. This canproduce a relatively narrow viewing angle in the referenced viewingplane, where the degree of collimation is tailored at least in part bycontrolling the magnitude of the z-axis refractive index differencebetween microlayers in the collimating birefringent reflectivepolarizer.

Thus, the increase in reflectivity with incidence angle in thecollimating birefringent reflective polarizer can refer to light of auseable polarization state incident in a plane for which oblique lightof the useable polarization state is p-polarized for an incidence plane,and s-polarized for the orthogonal incidence plane. Alternately, suchincrease in reflectivity can refer to the average reflectivity ofunpolarized light in any plane of incidence.

In many embodiments, the collimating birefringent reflective polarizersof a recycling cavity or film stack, also have a sloped transmissionspectrum, and often a blue-sloped transmission spectrum for lightincident in both planes of incidence for either a usable polarizationstate, or for unpolarized light in any plane of incidence.

Referring back to FIG. 1, an absorbing polarizer 104 can be stackedbetween the collimating birefringent reflective polarizer 102 and firstbirefringent reflective polarizer 106, and potentially laminated to oneor more of the reflective polarizers, co-extruded with one or more ofthe reflective polarizers or coated onto and oriented with one or morereflective polarizers. In some exemplary embodiments, the entire stack100 illustrated in FIG. 1 may be coextruded as a single film, or partsof it can be separately extruded and laminated, or first oriented andthen laminated.

Consider an ARCP located between a recycling backlight and the glasssurface upon which resides the pixilated Liquid Crystal Display (LCD)structure(s), where the usual absorbing display polarizer of the LCDthat faces the recycling backlight, is removed. The spectraltransmission and reflection properties of an ARCP can conveniently bedefined by its hemispheric reflectivity spectrum, R^(f) _(hemi)(λ), andby its transmission spectrum for light polarized along either the passaxis of the LCD system T^(pass)(Ω,λ) and for light linearly polarizedalong the orthogonal, block axis of the LCD system, T^(block)(Ω,λ).

The ARCP films of this invention, when used in a recycling backlight,can provide for brightness enhancement of the white state of the LCD andsimultaneously provide a low intensity dark state for the LCD.

More generally, the desired ARCP property of providing enhancedintensity of light passage through white-state LC cells and overlyingpolarizer, and near zero intensity of light passage through dark-stateLC cells and overlying polarizer, should be controlled across thevisible spectrum so that the perceived color of the liquid crystaldisplay is not biased or distorted by the ARCP/backlight system. If theARCP/backlight system delivers light to the LCD that is inconsistentwith the engineered chromaticity balance of the display light source(e.g. compact fluorescent bulbs or a display-grade white LED) and thecolor filters in each LC cell, then the LCD image will have distortedcolor hues and white point values, and in addition, can have low-lightimage-areas (black or very dark areas of an image) that have a coloredhue, such as a bluish tint or a magenta, or reddish tint.

Absorbing polarizers, such as absorbing polarizer 104, are also suitablefor use in the present disclosure. One useful polarizing absorptiveelement is an oriented, dye-containing, polyvinyl alcohol (PVA) film.Examples of such films and their use as polarizing absorptive elementsare described, for example, in U.S. Pat. Nos. 4,895,769, and 4,659,523and PCT Publication No. WO 95/17691, all of which are incorporatedherein by reference. To function as an absorbing polarizer, thepolyvinyl alcohol film is typically stretched to orient the film. Whenstained with a polarizing dye or pigment, the orientation of the filmdetermines the optical properties (e.g., the axis of extinction) of thefilm. Preferably, the absorbing element is such that absorption of lightpolarized along the block axis does not decrease (and sometimesincreases) with increased angle of incidence. One example of suchabsorbing polarizers are those polarizers including supra-molecularlyotropic liquid-crystalline material, as described in Lazarev et al.article, entitled “Low-leakage off-angle in E-polarizers”, Journal ofthe SID 9/2, pp. 101-105 (2001), incorporated by reference herein.

Suitable IR and visible absorbing dyes utilized in the absorbingpolarizer include dyes with good thermal stability that can bemelt-processed with, for example, polyesters, e.g. PEN. The selection ofthe light absorbing material can be made based on factors, such as, forexample, the absorbance spectrum of the light absorbing material, cost,processability, stability, and compatibility with other elements in theoptical filter.

A light absorbing material may be selected such that the material has adichroic ratio of at least 5:1, 10:1, or potentially even 20:1. Dichroicratio may generally be understood as the ratio of the absorptionconstant in the block polarization to the absorption constant in thepass polarization. It will be appreciated that many light absorbingmaterials suitable for broadband absorptive elements have substantialabsorbance over a relatively wide range of wavelengths or a relativelyconstant absorbance value over portions of both the transmission andreflection wavelength ranges. The use of the combination of an absorbingpolarizer between two reflective polarizers can allow the use of lowerloadings of light absorbing material than if the absorptive element wasused alone or with a single reflective element.

Absorbing polarizers used in exemplary embodiments of the presentdisclosure have a contrast ratio of less than 1000:1, thus making thecontribution of the reflective polarizers more important. In someexemplary embodiments, the contrast ratios of absorbing polarizers maybe about 500:1 or less, about 100:1 or less, about 10:1 or less, orabout 5:1 or less. In some exemplary embodiments, the absorbingpolarizer may be characterized by a contrast ratio of about 5:1 to about100:1. One of skill in the art will understand that contrast ratio maygenerally be understood as the ratio of the percentage of transmittedlight polarized parallel to the pass axis to the percentage oftransmitted light polarized parallel to the block axis.

Where the absorbing polarizer of the reflective/absorbing/reflectivepolarizer combination has a contrast ratio of up to about 10:1, at leastone of the reflective polarizers preferably has a contrast of at leastabout 100:1. In other exemplary embodiments, one or both of the biaxialreflective polarizers may be characterized by a contrast ratio of atleast about 50:1, at least about 100:1 or at least about 200:1. Thepolarizer stack combination (such as stack 100 of FIG. 1) according tothe present disclosure may have a total contrast ratio of about 500:1 ormore or about 1000:1 or more. In at least one embodiment, the contrastratio of the polarizer stack (or ARCP) may be 5,000:1 or more, orpotentially 6,000:1 or more. In some exemplary embodiments, the contrastratio of the polarizer stack according to the present disclosure may beas high as about 10,000:1.

Referring back to FIG. 1, the third element of polarizer stack 100 notyet described in detail is first birefringent reflective polarizer 106.Any number of appropriate birefringent reflective polarizers may beemployed. For example, FIG. 3 illustrates one potential exemplaryembodiment of a reflective polarizer according to the presentdisclosure, which is a multilayer optical film 111 that includes a firstlayer of a first material 113 disposed (e.g., by coextrusion) on asecond layer of a second material 115. The depicted optical film 111 maybe described with reference to three mutually orthogonal axes x, y andz. Two orthogonal axes x and y are in the plane of the film 111(in-plane, or x and y axes) and a third axis (z-axis) extends in thedirection of the film thickness. One or both of the first and secondmaterials may be birefringent.

While only two layers are illustrated in FIG. 3 and generally describedherein, typical embodiments of the present disclosure include two ormore of the first layers interleaved with two or more of the secondlayers. The total number of layers may be hundreds or thousands or more.In some exemplary embodiments, adjacent first and second layers may bereferred to as an optical repeating unit. Reflective polarizers suitablefor use in exemplary embodiments of the present disclosure are describedin, for example, U.S. Pat. Nos. 5,882,774, 6,498,683, 5,808,794, whichare incorporated herein by reference.

The optical film 111 may include additional layers. The additionallayers may be optical, e.g., performing an additional optical function,or non-optical, e.g., selected for their mechanical or chemicalproperties. As discussed in U.S. Pat. No. 6,179,948, incorporated hereinby reference, these additional layers may be orientable under theprocess conditions described herein, and may contribute to the overalloptical and/or mechanical properties of the film, but for the purposesof clarity and simplicity these layers will not be further discussed inthis application.

In a birefringent reflective polarizer, the refractive indices of thefirst layers 113 (n_(1x), n_(1y), n_(1z)) and those of the second layers115 (n_(2x), n_(2y), n_(2z)) are substantially matched along onein-plane axis (y-axis) and are substantially mismatched along anotherin-plane axis (x-axis). The matched direction (y) forms a transmission(pass) axis or state of the polarizer, such that light polarized alongthat direction is preferentially transmitted, and the mismatcheddirection (x) forms a reflection (block) axis or state of the polarizer,such that light polarized along that direction is preferentiallyreflected. Generally, the larger the mismatch in refractive indicesalong the reflection direction and the closer the match in thetransmission direction, the better the performance of the polarizer.

To function well for wide angle viewing of a display device, a displaypolarizer should maintain high block state contrast for all angles ofincidence and also maintain high pass transmission over all angles ofincidence. As it has been shown in the commonly owned U.S. Pat. No.5,882,774, pass state transmission increases when the refractive indicesof the alternating first and second layers 113 and 115 are matched forlight polarized along the z-axis and for light polarized along they-axis. The z-index matching also ensures that the block statetransmission does not degrade at high angles of incidence.

In exemplary embodiments, at least one of the collimating birefringentreflective polarizer or first birefringent polarizer in a polarizerstack may be biaxial, that is, having Δn_(yz) of more than about 0.05for a particular birefringent polarizer material. In other exemplaryembodiments, Δn_(yz) can be at least 0.08 or another suitable valuedepending on the application or at least about 0.1 or greater. Allvalues of refractive indices and refractive index differences arereported for 633 nm.

In at least some embodiments, the first birefringent reflectivepolarizer 106 on the opposite side of absorbing polarizer 1104 fromcollimating birefringent reflective polarizer 102 may in fact itselfalso be a collimating reflective polarizer. Therefore, firstbirefringent reflective polarizer 106 may have all of the propertiesdiscussed above with respect to collimating birefringent reflectivepolarizer 102.

Yet another example of an appropriate polarizer stack according to thepresent description is illustrated in FIG. 4. Like polarizer stack 100,polarizer stack 400 includes a first birefringent reflective polarizer106, and a collimating birefringent reflective polarizer 102. Absorbingpolarizer 104 is positioned between first birefringent reflectivepolarizer 106 and collimating birefringent reflective polarizer 102.Additionally, the embodiment illustrated in FIG. 4 contains a secondabsorbing polarizer 108 that is positioned on the opposite side of firstbirefringent reflective polarizer 106 from the first absorbing polarizer104. Generally, the second absorbing polarizer 108 may have similar oridentical properties to those of absorbing polarizer 104, described indetail above. It may be particularly useful to utilize a secondabsorbing polarizer where one is seeking increased dark-stateextinction, or in instances where one is seeking to reduce glare fromthe viewer side of a display.

The present day LCD industry is focused on both high performance and lowcost products. A high performance absorbing, reflecting and collimatingpolarizer stack can be made by laminating separately manufactured highperformance iodine dyed PVA polarizers with the appropriate collimatingreflective polarizer, such as those described above. However, the highperformance iodine dyed polarizers are costly, and furthermore theirorientation direction, i.e. block axis direction, is typically in thedownweb direction (machine or MD direction) whereas oriented multilayerPEN based reflecting polarizers are most conveniently oriented in thecrossweb direction (transverse or “tenter” direction). For this reasonthey cannot be continuously joined together with a low cost roll-to-rolllamination method, but at least one must be first cut into piece partswhich are then rotated 90 degrees and then laminated individually to theother polarizer film. PVA coatings can be applied to a multilayerpolarizer film before orientation of the multilayer in a tenter, but wehave found that the large difference in the preferred orientationtemperatures of PVA and PEN result in poor quality of one or the otherof the absorbing and reflective components. A lower performing and lowercost iodine/PVA polarizer can be utilized in the present invention inplace of the alternate dyes that cited in this description, but thissubstitution does not solve the problem of the cost of the laminationprocess. Thus, there is still a need in the art for a low cost, highperformance absorbing reflecting collimating polarizer. As disclosedherein, certain polarizing dyes can be compounded into polyester resinssuch as PEN and PET or their copolymers and then extruded and orientedat the temperatures preferred for processing the polyesters. In thismanner, low cost absorbing polarizers can be fabricated as an integralpart of the polarizer stack with no further processing.

We now discuss further the interaction of the various polarizers inpolarizer stacks 100, 400, or the like. As taught in commonly owned andassigned PCT Application No. PCT/US2012/060483 (“PCT/US2012/060483”), acollimating reflecting polarizer (such as polarizer 102) providesenhanced collimation if the spectrum is sloped, with decreasingtransmission as the wavelength increases. For purposes of thisdescription, we define this as a negative slope (as will be described ingreater detail below). Reflective polarizers with relatively largenegative slopes for the pass axis transmission were disclosed in U.S.61/549,588. When the pass axis exhibits a large slope, the block axiswill also exhibit a similar slope, although not exactly the same slopedue to different indices of refraction for the block versus pass axis,and also due to large differences in the absolute values of transmissionon the linear scale on which the transmission is based. A large negativeslope on the block axis transmission polarizers can result in asubstantial leak for blue light, which can substantially discolor thedark state of an LCD panel. The collimating polarizers ofPCT/US2012/060483 were used in a backlight for an LCD panel thatincluded high performance absorbing polarizers. The block axis ofstandard LCD panel polarizers are selected for minimal dark stateleakage and eliminate such a problem.

For the integrated reflective absorptive collimating polarizer stacksdisclosed herein, there will be no additional absorbing polarizer on thepanel (as is customary) to absorb any light leakage. The integratedpolarizer must provide both good dark state, i.e., a low block axistransmission at all visible wavelengths, as well as provide for asubstantial collimation effect when used in an LCD system with arecycling backlight. In addition, the pass axis must transmit acceptablelevels for white state brightness. For the latter reason, simplylowering the block state transmission by using more ¼ wave layers, or alower index of refraction for the isotropic low index material in thepolarizer is not acceptable because it has the detrimental effect ofalso reducing the pass axis transmission to unacceptably low levels. Thelow index material is somewhat constrained for collimating reflectivepolarizers since it preferably has an index of refraction that is midwaybetween the y and z indices of the high index birefringent layers.

We have found that an acceptable color balanced block state dark level,acceptable collimation, and an acceptable pass state transmission levelcan all be simultaneously achieved if the block transmission spectra ofthe first birefringent reflective polarizer 106 and of the collimatingreflective polarizer 102 have oppositely sloped spectra. The pass axistransmission spectrum of the collimating reflective polarizer can beflat or slightly negative.

As discussed in PCT/US2012/060483, portions of the infrared spectrumshift into the visible spectrum for light that is incident at obliqueangles on the collimating polarizer. The parts of the spectrum that areprimarily responsible for reflecting and effectively collimatingobliquely incident white light are the green, red and near IR portionsof the spectrum that is measured at normal incidence. For this reason,the slope of those portions of the spectrum are most pertinent, both forthe block and for the pass axis of the individual reflective polarizers.

For the polarizer stack as a whole, it is worthwhile to assess a slopedetermination for the visible spectrum only because the absorbing dyespectra do not shift with angle. Accordingly, the visible slopecalculation is also provided to characterize the “color neutrality” ofthe polarizer stack or ARCP as a whole.

The transmission spectra of multilayer polymeric interference filterssuch as collimating reflective polarizers can often appear to berelatively noisy, e.g. interference effects can result in many localminima and maxima due to oscillations in the spectra. However, there areseveral ways to estimate or gauge the average slope of a spectrum over abroad wavelength range. Curve fitting to linear or polynomial functionsof low order is one method. A simpler method is to determine averagetransmission values in local groups of wavelengths. For example, theaverage transmission in a broadband visible and IR spectrum can becalculated for subsets of the IR, red and green wavelength ranges:IR_(avg), R_(avg), and G_(avg). A spectrum slope can be then calculatedas:

Slope=(IR_(avg) −G _(avg))/AVG,

where AVG is the simple arithmetic average of the three wavelengthaverages given by:

AVG=(IR_(avg) +R _(avg) +G _(avg))/3.

Any discussion with respect to “slope” of a first birefringentreflective polarizer or collimating birefringent reflective polarizerwill be understood to refer to the slope calculation above (i.e. thattaking into account IR, Red and Green).

In another sense, one can determine slope of the spectra that willinclude only the visible spectrum. In such a case, the averagetransmission can be calculated for subsets of the blue, red and greenwavelength ranges: B_(avg), R_(avg), and G_(avg). A visible slope can bethen calculated as:

Visible Slope=(B _(avg) −R _(avg))/Visible AVG,

where Visible AVG is the simple arithmetic average of the threewavelength averages given by:

Visible AVG=(B _(avg) +R _(avg) +G _(avg))/3.

Any discussion with respect to “visible slope” of the polarizerstack/ARCP will be understood to refer to the slope calculationimmediately above (i.e. that taking into account Blue, Red and Green).

The local color ranges are selected as Blue=450 nm to 500 nm: Green=500to 560 nm, Red=600 to 650 nm and IR=700 to 750. One of skill in the artwill understand that slope may be expressed as either a fraction or apercentage.

For purposes of this description, a “neutral” visible slope will bedefined as having an absolute value of 15%, 10%, 5% or less, orpotentially 3% or less, and potentially even close to 0. The polarizerstack of the present description or ARCP may generally be understood tohave a neutral visible slope for transmitted light (i.e., the pass axistransmission of the polarizer stack as a whole is substantially neutralacross the visible wavelength band).

In the present description, the slope of the transmission spectrum inthe pass state (i.e. T^(pass)(0,λ)) for the collimating birefringentreflective polarizer may be less than 0 and less than −5% or less than−10% or less than −20% or less than −30% or less than −40% or less than−50%. The slope of the transmission spectrum in the block state (i.e.T^(block)(0,λ) may be less than 0 or less than −10% or less than −50% orless than −100% or less than −150% or less than −200%.

The slope of the transmission spectrum in the pass state (i.e.T^(pass)(0,λ)) for the first birefringent reflective polarizer may begreater than 0 and greater than 5% or greater than 10% or greater than20% or greater than 30% or greater than 40% or greater than 50%. Theslope of the transmission spectrum in the block state (i.e.T^(block)(0,λ) may be greater than 0 or greater than 10% or greater than50% or greater than 100% or greater than 150% or greater than 200%.

An indication of the potential degree of collimation of light from apolarizer stack can be obtained from the ratio of transmitted visiblelight at normal incidence and 60 degrees incidence of a given film. Inmost LCD TVs, the rear polarizer on the LCD panel is aligned with itspass axis in the horizontal direction. Thus the plane of incidence ofp-polarized pass axis light is along the horizontal direction (left andright). The plane of incidence of s-polarized pass axis light istherefore in the vertical direction. Thus it is the backlight emissionof s-polarized light that determines the brightness of the LCD panelwhen viewed from above or below the centerline and the backlightemission of p-polarized light that determines the viewing brightnessfrom the left or the right. For this reason, the pass axis spectra ofthe three film examples are shown for s-polarized and for p-polarizedlight at 60 degrees and at normal incidence.

In the present description, visible light transmitted in the pass axisat 60 degrees incidence (“T^(pass)60”) may be divided by lighttransmitted in the pass axis at 0 degrees incidence (“T^(pass)60”) todetermine level of collimation. Such measurements may be taken for boths-polarized and p-polarized light. The currently described polarizerstacks exhibit with s-polarized light, a ratio of T^(pass)60/T^(pass)0that is less than 0.75 and further less than 0.60. Similarly, withp-polarized light the currently described polarizer stacks exhibit aratio of T^(pass)60/T^(pass)0 that is less than 0.75 and further lessthan 0.60. Additionally, one can characterize the polarizer stack interms of the visible light transmitted in the block axis at 60 degrees(“T^(block)60”) or at 0 degrees (“T^(block)0”). In preferred embodimentsof the present description the polarizer stack will have a T^(block)0that is very low, potentially less than 10⁻³ or even 0.5×10⁻³ or lessthan 10⁻⁴.

Recycling Backlight with an ARCP Front Element

For illustrative purposes, it is convenient to further define theoptical elements of the backlight with a front reflector and backreflector, forming the recycling cavity, and the Liquid Crystal Displaypanel. In some cases, the present description actually relates to arecycling backlight that may include the polarizer stack or ARCP asdescribed throughout in addition to a light source. The description mayalso encompass a display that includes a panel and such a backlight.

FIG. 7 is a schematic cross-section view of a display system 700 thatincludes a backlight 710 and an LC panel 730. The backlight 710 ispositioned to provide light to the LC panel 730. The backlight 710includes a front reflector 712 and a back reflector 714 that form alight recycling cavity 716 having a cavity depth H and an output region718 of area Aout. The front reflector 712 may have other elementsdisposed between it and the back reflector 714, for instance, variouslight control films, such as micro-structured lenslet arrays, prismaticfilms and beaded gain diffusers 711. Any suitable films described hereincan be utilized to provide the front reflector 712. Generally, the frontreflector will be an ARCP.

The LC panel 730 typically includes a layer of LC 736 disposed betweenpanel plates 738. The plates 738 are often formed of glass and mayinclude electrode structures and alignment layers on their innersurfaces for controlling the orientation of the liquid crystals in theLC layer 736. These electrode structures are commonly arranged so as todefine LC panel pixels, i.e., areas of the LC layer where theorientation of the liquid crystals can be controlled independently ofadjacent areas. A color filter array 740 may also be included with oneor more of the plates 738 for imposing color on the image displayed bythe LC panel 730.

The LC panel 730 is positioned between a front absorbing polarizer 732and front reflector 712. It is common practice in the art to positionthe LC panel 730, between a front and a rear absorbing polarizer, but inthis embodiment, the rear absorbing polarizer is replaced by anabsorbing, reflecting, collimating and polarizing element, 712, alsodescribed as the front reflector (of the recycling backlight). It isunderstood that the front reflector 712, may be directly attached to theglass plate 738. The absorbing polarizer 732, the front reflector (ARCP)712, and the LC panel 730 in combination control the transmission oflight from a backlight 710 through the display system 700 to the viewer.For example, the absorbing polarizer 732, and the ARCP 712, may bearranged with their pass transmission axes perpendicular to each other.In an unactivated state, a pixel of the LC layer 736 may not change thepolarization of light passing therethrough. Accordingly, light thatpasses through the ARCP 712 is absorbed by the front absorbing polarizer732. When the pixel is activated, the polarization of the light passingtherethrough is rotated so that at least some of the light that istransmitted through the ARCP 712 is also transmitted through the frontabsorbing polarizer 732. Selective activation of the different pixels ofthe LC layer 736, for example, by a controller (not shown), results inthe light passing out of the display system 700 at certain desiredlocations, thus forming an image seen by the viewer. The controller mayinclude, for example, a computer or a television controller thatreceives and displays television images.

One or more optional layers (not shown) may be provided proximate thefront absorbing polarizer 732, for example, to provide mechanical and/orenvironmental protection to the display surface. In one exemplaryembodiment, the layer may include a hardcoat over the front absorbingpolarizer 732.

It will be appreciated that some types of LC displays may operate in amanner different from that described above. For example, the absorbingpolarizers 732, and the ARFC 712, may be aligned parallel and the LCpanel may rotate the polarization of the light when in an unactivatedstate. Regardless, the basic structure of such displays remains similarto that described above.

For analysis purposes in which we consider the front and back reflectorto be of substantially infinite extent, we can define a back reflectoreffective reflectivity for visible unpolarized light, R_(BL), asincluding all of the reflective and loss elements within the interior ofthe recycling cavity other than the aperture defining the outputsurface. In this regard, loss elements such as LED dies, lenses,packaging, circuitry, and exposed circuit board, are included in anarea-fraction sense, with the surrounding high-reflectivity materials,to determine R_(BL). Further, physical gaps between reflective surfacesare also included in defining this effective reflectivity. The physicallocation of this R_(BL) surface can then be conveniently drawn ascoincident with the mean surface of the physical cavity interior. Often,for well constructed recycling cavity backlights, the reduction ofR_(BL) from the measured value(s) of R^(b) _(hemi) for the backreflector material, is a few percent or less and will therefore beignored here.

Further, it is convenient to define the optical properties of the ARCPusing the simple constructs R^(f) _(hemi)(λ), T^(pass)(Ω, λ) andT^(block)(Ω, λ): T^(pass)(Ω, λ) is the transmission of light polarizedalong either the pass axis of the LCD system, T^(block)(Ω,λ) is thetransmission of light linearly polarized along the orthogonal, blockaxis of the LCD system, Ω represents the solid angle of interestrepresenting a viewers' geometrical location relative to the backlightoutput surface. A particular value of Ω, can be represented by acombination of the defined plane of incidence (22 and 24), with theincidence angle θ.

Furthermore, it is convenient to define front reflector and backreflector properties R^(f) _(hemi)(λ) R^(b) _(hemi)(λ) and T^(pass)(Ω,λ) and T^(block)(Ω, λ), as wavelength specific spectra properties, oralternatively, as averages across the visible band, in which case theyare written as R^(f) _(hemi), R_(BL), and T^(pass)(Ω, λ) andT^(block)(Ω, λ). For the purposes of this invention, the visibleaverages are taken as the spectrum average values within the wavelengthrange of 450 nm to 650 nm. Further, where the visible spectrum isdiscussed in the present description, it should be understood to meanthe wavelength range from 450 nm to 650 nm.

R^(f) _(hemi)(λ) is a measurable quantity, describing the hemisphericalreflectivity of the ARCP. As noted, numerous other optical films andoptical elements may be disposed between the ARCP and the backlight backreflector element, such as prismatic films, microlens array films, andbeaded gain diffusers, to name a few. The elements may be laminated orspaced apart, but in general they operate together as a system torecycle light from the backlight light sources in order to eitherthoroughly mix the light within the cavity, or to collimate the light inthe cavity towards the normal angle, prior to interaction with theoverlaying ARCP/LCD panel. The optical elements below the ARCP caninclude diffusive elements such as diffuser plates, and surfacestructure diffusers, as well as refractive elements such as lenticularand/or prismatic films.

The values of T^(pass)(Ω, λ) and T^(block)(Ω,λ), are defined as atransmission coefficients: the ratio of the transmitted intensity at anangle centered on the viewer angle of interest, Ω (relative to the frontreflector plane), with the front reflector and an absorbing polarizeroverlaying an all-angle light source (e.g., an angle-mixed recyclingcavity), to the intensity at 0 degrees for only the absorbing polarizeroverlaying the all-angle light source. For this measurement, thepolarization properties of the front reflector are appropriately alignedwith the pass axis of the absorbing polarizer. T^(pass)(Ω,λ) andT^(block)(Ω,λ) spectra have been measured for the examples below using aPerkin Elmer L-1050 spectrophotometer.

The spectral transmission and reflection properties of an ARCP iscomprised of the individual spectral transmission and reflectionproperties of each of the optical components of the ARCP: a firstbirefringent reflective polarizer disposed nearest to the recyclingbacklight element, 106, an absorbing polarizer 104 and a collimatingbirefringent reflective polarizer 102 that is positioned on the oppositeside of the absorbing polarizer from first birefringent polarizer 106.Optionally, an additional absorbing polarizer may be disposed adjacentto the LCD structure.

It is understood in the art, that if the reflection and transmissionproperties of the MOF components of an ARCP are known, and the absorbingproperties of the dichroic absorbing elements are known, then anumerical combination of each of the individual elements will determinethe overall spectral transmission and reflection properties of the ARCP.

The relationships among the ARCP spectral transmission and reflectionproperties, and spectral light intensity of each of the orthogonallinear polarizations (pass and block) delivered by the ARCP/recyclingbacklight system to the overlying LCD, can be seen by referencing thefollowing relationships:

$\begin{matrix}{{{Pass}\mspace{14mu} {Intensity}\mspace{14mu} {{Spectrum}\left( {\Omega,\lambda} \right)}} = \frac{T^{pass}\left( {\Omega,\lambda} \right)}{1 - {{R_{BL}(\lambda)}*{R_{Hemi}^{f}(\lambda)}}}} & {{Equation}\mspace{14mu} 1} \\{{{Block}\mspace{14mu} {Intensity}\mspace{14mu} {{Spectrum}\left( {\Omega,\lambda} \right)}} = \frac{T^{block}\left( {\Omega,\lambda} \right)}{1 - {{R_{BL}(\lambda)}*{R_{Hemi}^{f}(\lambda)}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

For the construction illustrated in FIG. 7, the pass Intensity Spectrumand Block Intensity Spectrum may both be substantially neutral across avisible wavelength band and have the ratio of at least 1,000:1 at normalincidence.

EXAMPLES Materials

Abbreviation/ product name Description Available from PD-325H Dichroicdye Mitsui Fine Chemical, Japan PD-335H Dichroic dye Mitsui FineChemical, Japan PD-104 Dichroic dye Mitsui Fine Chemical, Japan PD-318HDichroic dye Mitsui Fine Chemical, Japan OCA 8171 Optically clearadhesive 3M Company, St. Paul, MN APF Advanced Polarizer Film 3MCompany, St Paul, MN Sanritz-5618 Linear absorbing Sanritz Corporation,Tokyo, polarizer available under Japan. the trade name HLC2-5618 GN071PETg PETg skins Eastman Chemical, Kingsport, TN PEN pellets Polyethylenenaphthalate 3M Company, St Paul, MN resin pellets

Fabrication of Absorbing Polarizer

An absorbing polarizer layer was formed using the following process. PENpellets were fed into a twin screw extruder at a rate of 22.7 kg/hr(50.1 pounds per hour (pph)). Dichroic dyes purchased from MitsuiChemical were also fed into the twin screw extruder at the followingrates: PD-104: 45 g/hr (0.10 pph), PD-325H: 59 g/hr (0.13 pph), PD-335H:32 g/hr (0.07 pph), and PD-318H: 77 g/hr (0.17 pph). This mixture wasfed along with GN071 PETg skins at a rate of 22.7 kg/hr (50 pph) on eachside through a 41 cm (16 inch) die to form a cast sheet with a speed of6.1 m/min (20 feet per minute). The cast sheet was stretched in a tenterat a ratio of 5.6:1 at a temperature of 143° C. (290° F.) with a speedof 6.1 m/min (20 feet per minute).

Comparative Example C1

A hybrid polarizer was constructed by laminating the following stacktogether using optically clear adhesive OCA 8171 between layers: anabsorbing polarizer layer, APF (advanced polarizer film a multilayeroptical film), a second absorbing polarizer layer, and a second APFlayer. The absorbing polarizer layers were as described in “Fabricationof Absorbing Polarizer”.

A Westinghouse LD-3240 model television was obtained. The film stackbehind the LCD panel in the TV contained a polarizer, a cover sheet, adiffuser sheet and a prism film. These films were removed from the LCDpanel and used to assemble the stacks indicated in Table 1. The 90degree (azimuthal) luminance of the LCD panels with the film stacksindicated in Table 1 was measured as a function of polar angle using aEZ contrast XL 88W conoscope (Model XL88W-R-111124, available fromEldim-Optics, Herouville, Saint-Clair France). The luminance data isshown in FIG. 6.

TABLE 1 Polar Angle at Maximum Maximum Luminance Luminance PolarizerStack (degrees) (nits) Hybrid polarizer + cover sheet + 25 171 diffusersheet Hybrid polarizer + cover sheet + 6 211 prism film + diffuser sheetSanritz-5618 polarizer + cover sheet + 6 175 prism film + diffuser sheet

Comparative Example C2

A Collimating birefringent reflective polarizer was prepared as follows.Two multilayer optical film packets were coextruded as described inExample 3 of PCT Patent Application Number US 2012/060485 except that325 layers were used in each packet and an absorbing polarizer layer wascoextruded at the top surface. The absorbing polarizer layer was madefrom the materials described above under the header “Fabrication ofAbsorbing Polarizer.”

Example 1

The ARCP polarizer of FIG. 4 was manufactured using the feedblock methoddescribed in U.S. Patent Application Publication No. 2011/0272849entitled “Feedblock for Manufacturing Multilayer Polymeric Films”, filedMay 7, 2010. Two packets of alternating low and high index polymerlayers were coextruded as a cast web and then stretched in a tenter on acontinuous film making line. Two absorbing polarizer layers werecoextruded along with the two packets with one absorbing polarizer layerplaced between the two packets and the other placed at the top of theARCP stack. The absorbing polarizer layers were made from the resindescribed above under the header “Fabrication of Absorbing Polarizer.”The first packet and the second packet were each a stack of 325 layerswhere the high index layer was constrained uniaxial PEN and the lowindex layer was GN701 PETg. The packets were designed so that the firstpacket was a reflective polarizer and the second packet was acollimating reflective polarizer.

The contrast ratio was determined as the ratio of average passtransmission to average block transmission. The ratio of pass axisp-polarized transmission at the polar angle of 60 degrees to the normalincidence pass axis transmission was also determined A low transmissionratio indicated a strong collimation effect. The results are reported inTable 2 where it can be seen that the ARCP polarizer of Example 1, incontrast to both of the comparative examples, simultaneously gave a lowT^(pass)(60 p-pol)/T^(pass)(0) transmission ratio and a high contrastratio.

TABLE 2 Polarizer T^(pass)(60 p-pol)/T^(pass)(0) Contrast Ratio Hybridpolarizer (Comparative 0.95 5170 Example C1) CMOF polarizer (Comparative0.54 10 Example C2) ARCP polarizer (Example 1) 0.57 6755

An LG Flatron IPS231P monitor was obtained. The film stack behind theLCD panel in the monitor contained a polarizer, a prism film and amicrolens film. These films were removed from the LCD panel and used toassemble the stacks indicated in Table 3. The 90 degree (azimuthal)luminance of the LCD panels with the film stacks indicated in Table 3was measured as a function of polar angle using a EZ contrast XL 88Wconoscope (Model XL88W-R-111124, available from Eldim-Optics,Herouville, Saint-Clair France). The luminance data is shown in FIG. 7.

TABLE 3 Maximum Polar Angle at Maximum Luminance Polarizer StackLuminance (degrees) (nits) ARCP polarizer + microlens  9 200 film ARCPpolarizer + prism film + About 0 225 microlens film Sanritz-5618polarizer + 14 125 microlens film Sanritz-5618 polarizer + prism About 0172 film + microlens film

Example 2a

An absorbing, reflective collimating polarizer of FIG. 4 was fabricated,both without, and with, the dichroic absorbing elements 108 and 104 ofFIG. 4. Consider first, the instance where there was no dichroicabsorbing element between or adjacent to the first and second reflectivepolarizers. The first birefringent polarizer was a stack of 325 layers,where the high index layer is constrained uniaxial PEN and the low indexmaterial is PETg., and was disposed nearest to the liquid crystaldisplay. The second, collimating, birefringent reflective polarizer wasdisposed facing (nearest to) the recycling backlight. This secondcollimating reflective polarizer was also a stack of 325 layers, wherethe high index layer is constrained uniaxial PEN and the low index isPETg. Each of the first and second reflective polarizers was configuredand processed to provide an appropriate pass transmission spectrum andblock transmission spectrum.

The measured block transmission spectra for each of the first and secondreflective polarizers is shown in FIG. 8 a. Also shown in FIG. 8 a arethe computed block transmission spectra for each of the first and secondreflective polarizers. The computed spectra were obtained using AtomicForce Microscopy (AFM) measurements of each of the 325 layer structures,the measured dispersive refractive index values for each of the 3orthogonal axes for the PEN and PETg multilayer materials, and employinga 4×4 Berreman Matrix computation engine for a layered opticalstructure. The computed spectra for the block polarization agreed verywell with the measured spectra. This method of employing computedoptical spectra, employing input from a combination of AFM anddispersive refractive index values, has been demonstrated to be bothaccurate and predictive, to those skilled in the art.

FIG. 8 b shows the measured and computed pass spectra for the first andsecond reflective polarizers of this example. Again, the agreement wasvery good.

The computed slopes for each of the first and second reflectivepolarizer elements are shown in Table 4 below.

TABLE 4 T^(pass)(0, λ) 1st T^(block)(0, λ) 1st T^(pass)(0, λ) 2ndT^(block)(0, λ) 2nd Refl Pol Refl Pol Refl Pol Refl Pol MeasuredMeasured Measured Measured slope  5% 39% −3% −81% T^(pass)(0, λ) 1stT^(block)(0, λ) 1st T^(pass)(0, λ) 2nd T^(block)(0, λ) 2nd Refl Pol ReflPol Refl Pol Refl Pol Computed Computed Computed Computed slope −1% 59%−9% −88%

In summary, the slope for T^(block)(0,λ) for the first reflectivepolarizer was strongly positive (sloped to higher transmission towardthe red) and T^(block)(0,λ) for the second reflective polarizer wasstrongly negative (sloped to higher transmission toward the blue). Thecalculated slope for T^(pass)(0,λ) for the first reflective polarizerwas “neutral”, with close to a zero slope, and the slope forT^(pass)(0,λ) for the second, collimating reflective polarizer wasmodestly negative (sloped to higher transmission toward the blue).

FIG. 8 c shows the transmission spectra for element 108 of FIG. 4, wherethe absorbing polarizer layers 108 and 104 were formed in conjunctionwith the first reflective polarizer and the second collimatingreflective polarizer described above and illustrated in FIGS. 8 a and 8b. The absorbing polarizer layers were formed using the processdescribed above, and were separated from the ARCP in order for thespectra T^(block)(0,λ) and T^(pass)(0,λ) to be measured and subsequentlybe computed, by fitting the absorption constants of the dichroic dyes,to match the spectral data. The dichroic ratio for the dyes in thisexample had an average value across the visible spectrum of about 6.5,and showed a slightly positive slope for T^(pass)(0,λ) and a neutralslope for T^(block)(0,λ).

FIG. 8 d, shows the measured and computed transmission spectra for theoverall ARCP of FIG. 4, where the absorbing polarizer layer 104 wasdisposed between the first and second reflective polarizers, and 108 wasadjacent to the first reflective polarizer.

FIG. 8 d shows good agreement between the measured and the computedspectra T^(block)(0,λ) and T^(pass)(0,λ), and it can be seen that thetransmission for T^(block)(0,λ) was both very low, approximate 10⁻⁴across the visible wavelength band, and was substantially neutral. Atthe same time, the level of collimation and the level of blocktransmission provided by the ARCP are shown in Table 5, where visibleaverages are used for the collimation metrics.

TABLE 5 T^(pass)(60 p-pol)/ T^(pass)(60 s-pol)/ T^(pass)(0) T^(pass)(0)T^(block)(0) 0.672 0.486 1.04e−04A direct comparison of the measured T^(block)(0,λ) and T^(pass)(0,λ)spectra for the fabricated ARCP, and a standard absorbing displaypolarizer, 732 of FIG. 7, is shown in FIG. 9. The curves annotated withsymbols are for the standard absorbing display polarizer, in thisinstance a Sanritz 5618, and the solid lines show T^(block)(0,λ) andT^(pass)(0,λ) spectra for the fabricated ARCP of this example.

Using the backlight relationships discussed above, we then calculatedthe backlight intensity spectra, using the spectra of R_(BL)(λ), R^(f)_(hemi)(λ), T^(pass)(Ω, λ) and T^(block)(Ω, λ). We next analyzed thebacklight Pass Intensity Spectra and backlight Block Intensity Spectra,in order to determine color outcome in both the white state and the darkstate, and the ratio of the two. For this example, we took R_(BL)(λ) tobe constant with wavelength, and had a value of 0.87.

FIG. 10 shows the spectra of R_(BL)(λ) (curve A) for the backlight backreflector, and of the example ARCP R^(f) _(hemi)(λ) (curve B). Alsoplotted are backlight Pass Intensity spectra, calculated for the viewangles 0 degrees (curve C), 60 degrees for p-polarized light propagatingin the plane of incidence 122 of FIG. 2 (curve D), and 60 degrees fors-polarized light propagating in the plane of incidence 132 of 22 FIG. 2(curve E).

Finally, the backlight system of this example, with the ARCP frontreflector was analyzed for backlight emitted color, at the 0 degree andat the positive and negative 60 degree view angles, for each of thep-polarized and s-polarized pass axes (labeled 60S and 60P). Thechromaticity data for backlight Pass and Block Intensity Spectra areshown in FIGS. 11 a and 11 b.

The pass state color for this example was very near the neutralbacklight color (determined by the LED spectrum and the color filters inthe LC panel) for both the normal-angle view, and for oblique angleviews. At the same time, the block state color at normal angle was alsonear the neutral color point, indicating that the display black pixelregions did not have a colored tint but would appear neutral black. Theblock state color coordinates differed from the backlight neutral colorpoint by no more than 0.035 for the y deviation, and 0.012 for the xdeviation. These levels of x and y chromaticity deviation would bedeemed neutral by those skilled in the art. In addition, the deviationof the block state color coordinates from the neutral backlight colorcoordinate was along the black-body temperature line, which runs throughthe (0.3, 0.3) chromaticity coordinate, from the upper right to thelower left.

The ratio of the visible-average pass Intensity Spectrum to thevisible-average Block Intensity Spectrum was about 4000:1 in thisinstance.

Example 2b

A second ARCP was generated, in which both the first and the secondreflective polarizer were configured in the same manner as Example 2aabove; each had the same spectral properties of a ˜−80% slope for theT^(block)(0, λ), and each had the same spectral properties of ˜−5% slopefor T^(pass)(0, λ). The slopes for the first and second reflectivepolarizers for this ARCP were tabulated in Table 6.

TABLE 6 T^(pass)(0, λ) 1st T^(block)(0, λ) 1st T^(pass)(0, λ) 2ndT^(block)(0, λ) 2nd Refl Pol Refl Pol Refl Pol Refl Pol ComputedComputed Computed Computed slope −9% −88% −9% −88%

FIG. 12 shows the computed transmission spectra for the overall ARCP ofthis example 2b, in the configuration of FIG. 4, where the absorbingpolarizer layer 104 was disposed between the first and second reflectivepolarizers, and 108 was adjacent to the first reflective polarizer. Theabsorbing polarizer layers 104 and 108 were the same as those providedin Example 2a.

It can be seen from FIG. 12 that the transmission spectrum forT^(block)(0,λ) was very low (approximate 10⁻⁴ across the visiblewavelength band) but was substantially negatively sloped, with highertransmission in the blue spectrum and lower transmission in the red andnear-IR. At the same time, the level of collimation and the level ofblock transmission provided by the ARCP are shown in Table 7, where thevisible averages were used for the collimation metrics.

TABLE 7 T^(pass)(60 p-pol)/ T^(pass)(60 s-pol)/ T^(pass)(0) T^(pass)(0)T^(block)(0) 0.650 0.495 1.33e−04

Again, using the backlight relationship discussed above, we calculatedthe backlight intensity spectra, using the spectra of R_(BL)(λ), R^(f)_(hemi)(λ), T^(pass)(Ω, λ) and T^(block)(Ω, λ). We then analyzed thebacklight Pass Intensity Spectra and backlight Block Intensity Spectra,in order to determine color outcome in both the white state and the darkstate, and the ratio of the two. For this example, we took R_(BL)(λ) tobe constant with wavelength, and have a value of 0.87.

FIG. 13 shows the spectra of R_(BL)(λ) (curve A) for the backlight backreflector, and of the Example 2b ARCP R^(f) _(hemi)(λ) (curve B). Alsoplotted are backlight Pass Intensity Spectra, calculated for the viewangles 0 degrees (curve C), 60 degrees for p-polarized light propagatingin the plane of incidence 122 of FIG. 2 (curve D), and 60 degrees fors-polarized light propagating in the plane of incidence 132 of 22 FIG. 2(curve E).

FIGS. 14 a and 14 b show the backlight system color response of thisExample 2b, at the 0 degree and at the positive and negative 60 degreeview angles, for each of the p-polarized and s-polarized pass axes(labeled 60S and 60P).

The pass state color for this Example 2b was again very near the neutralbacklight color for both the normal-angle view, and for oblique angleviews. However, for this Example 2b, the block state color at normalangle was far from the neutral color point, indicating that the displayblack pixel regions would appear a bluish hue. The block state colorcoordinates differed from the backlight neutral color point, by 0.178,for the y deviation and 0.225, for the x deviation. These levels of xand y chromaticity deviation were deemed non-neutral, and unacceptablefor a backlight and LC Display.

The ratio of the visible-average pass Intensity Spectrum to thevisible-average Block Intensity Spectrum was about 2600:1 in thisinstance.

Example 2c

A third ARCP was generated, in which both the first and the secondreflective polarizer were configured in the same manner as Example 2aabove; each had the same spectral properties of a ˜−80% slope for theT^(block)(0, λ), and each had the same spectral properties of ˜−5% slopefor T^(pass)(0, λ). The slopes for the first and second reflectivepolarizers for this ARCP, were tabulated in Table 8.

TABLE 8 T^(pass)(0, λ) 1st T^(block)(0, λ) 1st T^(pass)(0, λ) 2ndT^(block)(0, λ) 2nd Refl Pol Refl Pol Refl Pol Refl Pol ComputedComputed Computed Computed slope −1% 59% −1% 59%

FIG. 15 shows the computed transmission spectra for the overall ARCP ofthis example 2c, in the configuration of FIG. 4, where the absorbingpolarizer layer 104 was disposed between the first and second reflectivepolarizers, and 108 was adjacent to the first reflective polarizer. Theabsorbing polarizer layers 104 and 108 were the same as those providedin Example 2a.

It can be seen from FIG. 15, that the transmission spectrum forT^(block)(0,λ) was very low, less than 10⁻⁴ across the visiblewavelength band, but was substantially positively sloped, with lowertransmission in the blue spectrum and higher transmission in the red andnear-IR. At the same time, the level of collimation and the level ofblock transmission provided by the ARCP are shown in Table 9, wherevisible averages were used for the collimation metrics.

TABLE 9 T^(pass)(60 p-pol)/ T^(pass)(60 s-pol)/ T^(pass)(0) T^(pass)(0)T^(block)(0) 0.734 0.547 6.89e−05

Again, using the backlight relationships discussed above, we calculatedthe backlight intensity spectra, using the spectra of R_(BL)(λ), R^(f)_(hemi)(λ), T^(pass)(Ω, λ) and T^(block)(Ω, λ), and then analyzed thebacklight Pass Intensity Spectra and backlight Block Intensity Spectra,in order to determine color outcome in both the white state and the darkstate, and the ratio of the two. For this example, we again tookR_(BL)(λ) to be constant with wavelength, and have a value of 0.87.

FIG. 16 shows the spectra of R_(BL)(λ) (curve A) for the backlight backreflector, and of the Example 2c ARCP R^(f) _(hemi)(λ) (curve B). Alsoplotted are backlight Pass Intensity Spectra, calculated for the viewangles 0 degrees (curve C), 60 degrees for p-polarized light propagatingin the plane of incidence 122 of FIG. 2 (curve D), and 60 degrees fors-polarized light propagating in the plane of incidence 132 of 22 FIG. 2(curve E).

FIGS. 17 a and 17 b shows the backlight system color response of thisExample 2c, at the 0 degree and at the positive and negative 60 degreeview angles, for each of the p-polarized and s-polarized pass axes(labeled 60S and 60P).

The pass state color for this example 2c was again very near the neutralbacklight color for both the normal-angle view, and for oblique angleviews. However, for this example 2c, the block state color at normalangle was removed from the neutral color point towards a magenta,indicating that the display black pixel regions would appear as amagenta hue. The block state color coordinates differed from thebacklight neutral color point, by 0.052 for the y deviation, and −0.003for the x deviation. These levels of x and y chromaticity deviation weredeemed non-neutral (deviation of the radial distance for the backlightneutral point is greater than 0.025), and unacceptable for a backlightand LC Display.

In addition, the deviation of the block state color coordinates from theneutral backlight color coordinate, ran nearly perpendicular to theblack-body temperature line, which runs through the (0.3, 0.3)chromaticity coordinate, from the upper right to the lower left.Perpendicular chromaticity coordinate deviations from the black-bodytemperature line, are known to be significantly more perceptible as acolor deviation, than those that run parallel, or along the Black-bodycolor temperature line, such as is that case for Example 2a.

The ratio of the visible-average pass Intensity Spectrum to thevisible-average Block Intensity Spectrum was about 9400:1 in thisinstance.

The present invention should not be considered limited to the particularexamples and embodiments described above, as such embodiments aredescribed in detail in order to facilitate explanation of variousaspects of the invention. Rather, the present invention should beunderstood to cover all aspects of the invention, including variousmodifications, equivalent processes, and alternative devices fallingwithin the scope of the invention as defined by the appended claims.

The following are exemplary embodiments according to the presentdisclosure:

Item 1. A polarizer stack comprising: a first birefringent reflectivepolarizer having pass and block axis transmission spectra, a collimatingbirefringent reflective polarizer having a block axis transmittance thatdecreases with increasing wavelength, and an absorbing polarizer layerpositioned between the first birefringent reflective polarizer andcollimating birefringent reflective polarizer, wherein the pass axistransmission of the polarizer stack as a whole is substantially neutralacross the visible wavelength band.Item 2. The polarizer stack of item 1, wherein the block axistransmittance of the first birefringent reflective polarizer increasesas wavelength increases across the visible spectrum.Item 3. The polarizer stack of item 1, wherein the pass axistransmittance of the collimating birefringent reflective polarizer isneutral or decreases as wavelength increases across the visiblespectrum.Item 4. The polarizer stack of item 1, wherein the polarizer stacksatisfies: T^(pass)60/T^(pass)0<0.75 for p-pol light.Item 5. The polarizer stack of item 4, wherein the polarizer stacksatisfies: T^(pass)60/T^(pass)0<0.60 for p-pol light.Item 6. The polarizer stack of item 4 or 5, wherein the polarizer stacksatisfies: T^(block)0<10⁻³.Item 7. The polarizer stack of item 1, wherein T^(pass) of visible lightis greater than 0.3.Item 8. The polarizer stack of item 1, wherein T^(pass) of visible lightis greater than 0.4.Item 9. The polarizer stack of item 1, wherein T^(pass) of visible lightis greater than 0.5.Item 10. The polarizer stack of item 1, wherein the polarizer stacksatisfies: T^(pass)60/T^(pass)0<0.75 for s-pol light and wherein thepolarizer stack satisfies: T^(block)0<10⁻³.Item 11. The polarizer stack of item 6, wherein the polarizer stacksatisfies: T^(pass)60/T^(pass)0<0.60 for s-pol light.Item 12. The polarizer stack of item 7 or 8, wherein the polarizer stacksatisfies: T^(block)0<10⁻³.Item 13. The polarizer stack of item 1, wherein the contrast ratio ofthe absorbing polarizer layer is 100:1 or less.Item 14. The polarizer stack of item 1, wherein the contrast ratio ofthe polarizer stack is 6,000:1 or more.Item 15. The polarizer stack of item 1, wherein the R_(hemi) of firstbirefringent reflective polarizer is <0.50, and the R_(hemi) of thecollimating birefringent reflective polarizer is at least 0.60Item 16. The polarizer stack of item 1, further comprising a secondabsorbing polarizer layer positioned on the opposite side of the firstbirefringent polarizer from the absorbing polarizer layer.Item 17. A backlight, comprising a light source and the polarizer stackof item 1.Item 18. A display comprising a panel and the backlight of item 17.Item 19. A backlight comprising:

-   -   (1) a light recycling cavity, the light cavity comprising: a        front reflector, a back reflector, a pass Intensity Spectrum and        a Block Intensity Spectrum, wherein the front reflector is        partially reflective and comprises an ARCP; and    -   (2) one or more light source members disposed to emit light into        the light recycling cavity;    -   wherein the pass Intensity Spectrum and Block Intensity Spectrum        are both substantially neutral across a visible wavelength band        and have the ratio of at least 500:1 at normal incidence.        Item 20. The backlight of item 19, wherein the ARCP comprises a        first birefringent reflective polarizer having pass and block        axis transmission spectra, a collimating birefringent reflective        polarizer having a block axis transmittance that decreases with        increasing wavelength, and an absorbing polarizer layer        positioned between the first birefringent reflective polarizer        and collimating birefringent reflective polarizer.        Item 21. The backlight of item 20, wherein the pass axis        transmission of the ARCP is substantially neutral across the        visible wavelength band.        Item 22. The backlight of item 21, wherein the pass Intensity        Spectrum and Block Intensity Spectrum are both substantially        neutral across a visible wavelength band and have the ratio of        at least 1,000:1 at normal incidence.        Item 23. The backlight of item 19, wherein the ARCP satisfies:        T^(pass)60/T^(pass)0<0.75 for p-pol light.        Item 24. The backlight of item 23, wherein the ARCP satisfies:        T^(pass)60/T^(pass)0<0.60 for p-pol light.        Item 25. The backlight of item 19, wherein the ARCP satisfies:        T^(pass)60/T^(pass)0<0.75 for s-pol light.        Item 26. The backlight of item 25, wherein the ARCP satisfies:        T^(pass)60/T^(pass)0<0.75 for s-pol light.

What is claimed is:
 1. A polarizer stack comprising: a firstbirefringent reflective polarizer having pass and block axistransmission spectra, a collimating birefringent reflective polarizerhaving a block axis transmittance that decreases with increasingwavelength, and an absorbing polarizer layer positioned between thefirst birefringent reflective polarizer and collimating birefringentreflective polarizer, wherein the pass axis transmission of thepolarizer stack as a whole is substantially neutral across the visiblewavelength band.
 2. The polarizer stack of claim 1, wherein the blockaxis transmittance of the first birefringent reflective polarizerincreases as wavelength increases across the visible spectrum.
 3. Thepolarizer stack of claim 1, wherein the pass axis transmittance of thecollimating birefringent reflective polarizer is neutral or decreases aswavelength increases across the visible spectrum.
 4. The polarizer stackof claim 1, wherein the polarizer stack satisfies:T^(pass)60/T^(pass)0<0.75 for p-pol light.
 5. The polarizer stack ofclaim 4, wherein the polarizer stack satisfies: T^(block)0<10⁻³.
 6. Thepolarizer stack of claim 1, wherein T^(pass) of visible light is greaterthan 0.3.
 7. The polarizer stack of claim 1, wherein the polarizer stacksatisfies: T^(pass)60/T^(pass)0<0.75 for s-pol light and wherein thepolarizer stack satisfies: T^(block)0<10⁻³.
 8. The polarizer stack ofclaim 1, wherein the contrast ratio of the absorbing polarizer layer is100:1 or less.
 9. The polarizer stack of claim 1, further comprising asecond absorbing polarizer layer positioned on the opposite side of thefirst birefringent polarizer from the absorbing polarizer layer.
 10. Abacklight, comprising a light source and the polarizer stack of claim 1.